1. Field of the Invention
The present invention relates to a solution applying apparatus and a solution applying method used for a technique for applying a solution, such as wet etching, in a forming technique for a semiconductor device.
2. Description of the Related Art
Processes for applying a solution to a substrate are often performed when manufacturing semiconductor devices such as thin film transistors (TFTs) in which thin films of semiconductor, insulator and metal or the like are laminated and formed on a semiconductor substrate or glass substrate, such as semiconductor integrated circuits, active matrix type liquid crystal electro-optical devices and the like.
Commonly used solution applying methods include (1) a method for immersing (dipping) a substrate into a bath containing a solution in batch etching method or the like, (2) a method for applying a solution as a shower from a plurality of jets onto the upper surface of the substrate (a spray method), and (3) a method for applying a solution from a nozzle near the center of the rotating surface while the substrate is rotated (spin coating method).
The spin coating method (3) is suitable for so-called single wafer type processing with which it is possible to treat substrates uniformly, one at a time. However, in the method (3), since the substrate is rotated at 100 to 1000 rpm, a solution which is caused to drop or flow onto the rotating surface near the center moves at high speed toward the outside of the rotating surface. Therefore the moving speed of the solution on the substrate is much higher than the methods (1) and (2). That is, the method (3) has good solution circulation. Further, the moving speed of the solution at the peripheral portion of the rotating surface is higher than at the center portion of the rotating surface because of centrifugal force.
Conventionally, the following problems occurs due to the above fact. When spin etching is performed using an etching solution which has a large contact angle with respect to the resist, for example, hydrofluoric acid based etching solutions in a wet etching process, the etching solution does not adequately wet the surface to be etched, so that the etching solution forms balls and flows off. As a result, island-shaped etching irregularities form over the entire surface of the substrate. In particular, when aluminum mixed acid based etching solution is used, a portion which is not etched remains at the peripheral portion.
Thus the phenomenon occurs that etching is performed nonuniformly within a single substrate surface. This phenomenon becomes more marked as the fineness of the resist pattern width progresses. That is, since an etching solution does not penetrate uniformly into the fine portion, solution circulation is liable to be unsatisfactory.
Further, the phenomenon becomes more marked as the area of the substrate increases. In other words, if the area of the substrate increases, contact between the etching solution and the surface of the substrate is liable to be nonuniform. Also, irregularities also occur in the spray method (2).
In a liquid crystal display device, since several hundred thousand thin film transistors (TFTs) are formed on a single substrate such as quartz or glass in the same process, defect of one TFT causes defect of a substrate. Therefore, irregularity of the above etching causes a decrease in yield.
Also, a technique for selectively crystallizing an amorphous semiconductor material using a catalytic action of a metal element in producing a TFT using a resist (mask) is disclosed by Japanese Patent Published Application No. 6-319167. In the technique, a resist (mask) is formed on an amorphous silicon film, and then a solution containing a metal element such as nickel is added (applied). As a result, in an opening of a photoresist (mask), the metal element is selectively in contact with the surface of the amorphous silicon film. In this state, by heating with thermal annealing or laser light irradiation, the amorphous silicon film is crystallized from a region which the metal element is in contact with.
However, if the solution containing a metal element is uniformly added, since an amorphous silicon film is not crystallized sufficiently, characteristics of elements on the same substrate are different from each other.
Conventionally, in order to uniformly add (apply) a solution containing a metal element, an extremely thin oxide film is formed on the surface of an amorphous silicon film, so that wetting degree of the amorphous silicon film increases, or an interface active agent is added to the solution. However, this increases the number of process steps necessary to form the device.